The overall goal of this renewal continues to be the study of the integrin alpha6beta4 (referred to as beta4) as an approach for elucidating mechanisms involved in the genesis and progression of breast carcinoma, and for identifying targets for the clinical management of this disease. This integrin anchors basal epithelial cells to the basement membrane in inert structures termed hemidesmosomes (HDs). A seminal finding made in work funded by this grant is that beta4 is mobilized from HDs in invasive carcinomas and it translocates to the leading edge of cells where it engages F-actin and promotes migration/invasion. Elucidating the mechanism that regulates this mobilization of beta4 is essential for understanding its contribution to carcinoma biology. Also, much more needs to be learned about beta4 expression and function in human breast cancer. Indeed, a significant observation made with current funding is that beta4 expression correlates with basal-like tumors, aggressive tumors that retain features of basal epithelial cells and lack expression of ER, PR and HER2 (triple negative). Moreover, a beta4 gene signature has been generated, a cluster of 90 genes whose expression correlates significantly with beta4 in human breast tumors and that is prognostic for reduced survival and tumor recurrence. A new phase in the study of beta4 and cancer will be initiated based on the hypothesis that beta4 functions in concert with multiple proteins to drive a specific type of breast tumor and that beta4 is a powerful tool for understanding the contribution of these other proteins to breast cancer. The first aim will assess the hypothesis that PKC-alpha-mediated phosphorylation of three serine residues (S1356, S1360, S1364) in the beta4 intracellular domain triggers the mobilization of beta4 from HDs to F-actin and enables it to function in migration and invasion, and as a signaling receptor. This aim will be accomplished by generating a transgenic knock-in mouse in which these serines are mutated to alanines. Wound healing, beta4 signaling, and the genesis and progression of basal-like tumors will be assessed in these mice. The second aim will define the relationship between beta4 and actin-binding proteins that are members of the beta4 signature. Specifically, the actin-bundling protein fascin will be studied because its expression correlates with basal-like tumors and its functions could facilitate tumor progression. The contribution of fascin to the progression of breast tumors will be determined, and the hypothesis that beta4 regulates fascin localization in actin protrusions and that fascin is necessary for beta4-dependent migration and invasion will be assessed. The hypothesis that PKC-alpha regulates beta4 and fascin coordinately to drive migration and invasion will also be examined. The third aim focuses on SOX9, a transcription factor that is in the beta4 signature and the original basal-like gene cluster but whose functions in breast are unknown. The contribution of SOX9 to mammary gland development will be determined by generating a Sox9 targeted deletion in this epithelium. The involvement of SOX9 in basal-like tumors and its relationship to the beta4 signature will also be assessed.